Method for producing high cleanliness steel excellent in fatigue strength or cold workability

ABSTRACT

The present invention relates to a high-cleanliness steel having a high fatigue strength and high cold workability, and a method of making the high-cleanliness steel. The method adds a Li—Si alloy having a Li content between 20 and 40% and/or Li 2 CO 3  as a Li-containing substance to a molten steel. The Li-containing substance is added to the molten steel after the completion of a series of steps of a ladle refining process including composition adjustment, temperature adjustment and slag refining. The high-cleanliness steel has a total-Li content between 0.020 and 20 ppm by mass and contains 1.0 or below oxide inclusion particle having a major diameter of 20 μm or above at a maximum in 50 g of the steel wire. The steel contains an oxide inclusion that has a CaO content between 15 and 55%, SiO 2  content between 20 and 70%, an Al 2 O 3  content of 35% or below, a MgO content of 20% or below and a Li 2 O content between 0.5 and 20%. The high-cleanliness steel has improved fatigue characteristics and improved cold workability.

TECHNICAL FIELD

The present invention relates to a method of making a high-cleanlinesssteel having a high fatigue strength and high cold workability and, moreparticularly, to a method of making a high-cleanliness steel very usefulfor forming high-tension steel wires, very fine steel wires andhigh-strength springs, particularly, valve springs.

BACKGROUND ART

It is necessary to reduce, to the least possible extent, hardnonmetallic inclusions contained in steels for forming very fine steelwires whose diameters between 0.1 and 0.5 mm made by a cold wire drawingprocess and springs required to have high fatigue strength. Thenonmetallic inclusions cause the breakage of steel wires during wiredrawing and reduction of fatigue strength. Therefore, high-cleanlinesssteels containing the least unavoidable nonmetallic inclusions are usedfor the foregoing purposes.

Demand for weight reduction and output enhancement in automobiles hasbeen increased in recent years to reduce exhaust gases and to reducefuel cost. Therefore, high-stress designing of valve springs included inengines, and suspension springs included in suspension systems is arecent trend of design. Thus there is a tendency to increase thestrength of spring steels and to decrease the diameters of springs.Consequently, load stress becomes higher. Thus, there is a demand forhigh-performance steels having even more excellent characteristicsresistant to fatigue and sag. Valve springs are required, in particular,to have the highest fatigue strength.

The strength of very fine steel wires represented by those forming tirecords has been progressively increased to reduce the weight of tires.Recent steel cords have a strength on the order of 4000 MPa. Since veryfine steel wires having higher strength are more easily breakable duringcold working (wire drawing), the improvement of cold workability of suchsteel wires having high strength is desired.

As mentioned above, steel springs and very fine steel wires ofhigh-strength steels are more subject to fatigue fracture or breakagedue to nonmetallic inclusions contained in the steels. Thus the severityof demand for the reduction of nonmetallic inclusion content and sizehas been progressively increased.

A variety of techniques have been proposed for the reduction of hardnonmetallic inclusion content and size. Results of studies of preventingfatigue fracture are introduced in, for example, “126th and 127thNishiyama Memorial Technical Lecture”, The Iron and Steel Institute ofJapan, pp. 145-165, Nov. 14, 1988 (Reference 1). According to theresults of studies mentioned in Reference 1, the fatigue fracture ofspring steels does not occur when inclusions contained in the springsteels are those of a CaO—Al₂O₃—SiO₂ system having a melting pointbetween 1400 and 1500° C., and the reduction of nonductile inclusions,such as Al₂O₃, contained in tire cords is effective in preventingfatigue fracture. Means for making inclusions not detrimental areproposed in JP-B 6-74484 (Reference 2) and JP-B 6-74485 (Reference 3).Means mentioned in Reference 2 shows that inclusions are fractured anddispersed and become not detrimental during cold working or wire drawingwhen the composition of inclusions is 20 to 60% SiO₂, 10 to 80% MnO, 50%or below CaO and 15% or below MgO. Means mentioned in Reference 3 showsthat inclusions are fractured and dispersed and become not detrimentalduring cold working or wire drawing when the composition of inclusionsis 35 to 75% SiO₂, 30% or below Al₂O₃, 50% or below CaO and 25% or belowMgO. However, further improvement of the properties of steels isnecessary to meet recent quality requirement.

A technique proposed in JP-A 1-319623 (Reference 4) makes ahigh-cleanliness steel by mixing a mixture of a deoxidizer of a Sisystem and an alkaline metal compound in a molten steel to control thecomposition of the product of deoxidation such that the product containsthe alkaline metal. The alkaline metal decreases the melting point ofhard nonmetallic inclusions of Al₂O₃ and SiO₂ systems. The hardnonmetallic inclusions having a low melting point can be extended infilaments during hot rolling and become not detrimental to thedrawability and fatigue characteristics of the steel. Possible alkalinemetals are Na and Li, which have the same effect. The alkaline metalsingly added to the molten steel adversely affects the yield and henceit is recommended to use the alkaline metal together with thedeoxidizer. For example, LiF is added together with sodium silicate to apart, in which stirring Ar bubbles appear, of the molten steel pouredfrom a converter into a ladle at an initial stage of a ladle furnaceprocess (LF process).

A technique proposed in JP-A 2-15111 (Reference 5) adds an alkalinemetal to a molten steel to decrease the melting point of inclusions andto change the shape of inclusions during hot rolling. Possible alkalinemetals are Li, Na and K, which have the same effect. Since the alkalinemetal does not dissolve in the molten steel, it is recommended to use Sifor dilution. More concretely, a Si alloy containing 12% or below Li isused as a deoxidizer.

A technique proposed in JP-A 2002-167647 (Reference 6) adds an alkalinemetal oxide to the molten steel to improve the ductility of inclusionsincluding SiO₂ as a principal inclusion. According to Reference 6, theimprovement of the ductility of the inclusions is achieved by reducingthe energy of interface between the inclusions and the molten iron bythe alkaline metal instead of by decreasing the melting point asmentioned in References 3 and 4. In all cases, the alkaline metals Na, Kand Li are thought to be equivalent. The alkaline metal content of slagis on the order of 10% at a maximum. Practically, only Na is used.

A technique proposed in JP-A 2002-194497 (Reference 7) recommends usingan alkaline metal oxide for Si-deoxidation. This technique uses analkaline metal oxide because the alkaline metal oxide is able to reduceeffectively the activity of SiO₂ contained in ladle slag and,consequently, the oxygen content of the molten steel can be reduced.Possible alkaline oxides recommended by Reference 7 are Na₂O, K₂O andLi₂O, which have the same effect. The technique proposed in Reference 7differs from the technique proposed in Reference 5 in adding Li to themolten steel. More concretely, Li₂O in the form of a carbonate is mixedin slag and the Li content of the slag is on the order of 8% at amaximum.

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

Under the circumstance, the present invention is achieved and its objectis to provide a method of high-cleanliness steel having improved fatiguestrength and improved high cold workability, and the high-cleanlinesssteel produced by the method.

Inventors of the present invention made studies to solve the foregoingproblems and have found that Li has a particular effect unavailable fromother alkaline metals, such as Na and K. Although Li is the same as Naand K in the effect of decreasing the melting point of inclusions andonly Li is able to change greatly the properties of a multicomponentoxide inclusion, such as a multicomponent oxide expressed byCaO—Al₂O₃—SiO₂—MnO—MgO. The inventors have found that thischaracteristic effect of Li can be fully exhibited by adding Li to asteel by a proper method and the cold-workability and fatigue strengthof the steel can be remarkably improved.

More specifically, the inventors of the present invention have foundthat Li can be efficiently mixed in a molten steel by adding aLi-containing substance different from conventionally used Li-containingsubstances to the molten steel and the properties of a multicomponentoxide inclusion can be effectively changed by Li added to the moltensteel.

The techniques proposed in References 1 to 3 that adjust the compositionof an inclusion of a CaO—Al₂O₃—SiO₂ system so that the melting point ofthe inclusion may be between 1400 and 1500° C. are effective in reducingthe size of the inclusion particles to some extent. However, the sizediminishing effect of those techniques not utilizing the crystallizationpromoting effect of Li is insufficient. The object of those prior arttechniques is the direct control of the composition of the inclusion. Tocontrol the composition of the inclusion directly, it is important tomake harmful deoxidation products produced in the molten steel, such asSiO₂ and Al₂O₃, not detrimental by mixing not detrimental slag in themolten steel during slag refining and making the slag mixed in themolten steel combine and react with the harmful products. Although thisoperation does not reduce the total amount of oxygen greatly, dissolvedoxygen is reduced thermodynamically. Consequently, it is difficult forthe harmful deoxidation products, such as SiO₂ and Al₂O₃, to developwhen the molten steel solidifies. However, the molten steel and the slagmust be strongly stirred to control the composition of the inclusiondirectly by using the reaction of the slag. Consequently, the moltensteel is liable to contain inclusions originating in refractories.

Although the prior art techniques proposed in References 4 to 7 mentionLi, even the techniques proposed in References 3 to 6 areunsatisfactory. For example, the technique proposed in Reference 4 usesLiF, namely, a Li-containing substance, in combination with sodiumsilicate. However, LiF has a melting point of 842° C. and a boilingpoint of 1676° C., which is close to a steel-making temperature and theadding yield is insufficient. Therefore, the technique proposed inReference 4 needs to add LiF to a part, in which stirring Ar bubblesappear, of the molten steel poured from a converter into a ladle at aninitial stage of a ladle furnace process (LF process). Even if LiF isadded to the molten steel in this manner, it is difficult to add asufficient amount of Li to the molten steel, and the Li content of theslag increases excessively. The Li content of the slag actuallydetermined by the inventors of the present invention was as high as 4%.If the molten steel containing slag having a high Li content is stirredat the initial stage of the LF process, refractories are melted anddamaged and the amount of external inclusions originating in therefractories starts increasing. Moreover, Li becomes insufficient andthe inclusion diminishing effect of Li becomes insufficient.Consequently, the cold workability and fatigue characteristic of thesteel cannot be sufficiently improved.

The technique proposed in Reference 5 increases Li content in slag.Since a Si—Li alloy used by the technique proposed in Reference 5 has aLi content of 12% or below, the yield of Li is low. The Li content ofthe slag must be high to control inclusions with such a Si—Li alloy. Forexample, a second embodiment adds 700 kg of a Si—Li alloy having a Licontent of 2% containing 14 kg of Li to the slag of 240 t of a moltensteel during refining, and a third embodiment adds a Si—Li alloy havinga Li content of 5% containing 10 kg of Li to the slag of a molten steelduring refining. Even though the Si—Li alloy is added in such a manner,the Li content of the steel is insufficient and the Li content of theslag is high. Experiments conducted by the inventors of the presentinvention showed that the Li₂O content of the slag was between about 1%(second embodiment) and 1.5% (third embodiment). As mentioned inReference 4, the melting point and viscosity of the slag decreases, theamount of melted refractories increases and external inclusionsincreases even if the Li content of the slag is on the order of 1%.Moreover, since the amount of Li is insufficient, the size of inclusionscannot be satisfactorily diminished. Consequently, cold workability andfatigue characteristics cannot be satisfactorily improved.

According to References 6 and 7, the Li content of the slag is very highand a maximum Li content is between 8 and 10%. When the Li content ofthe slag is increased to such a high level, the Li content of the steelincreases to a permissible level, while the melting point and viscosityof the slag decrease considerably and refractories are meltedsignificantly. If such a slag is produced at an initial stage ofrefining and is stirred strongly, refractories are damaged severely andthe cold workability and fatigue characteristics of the steel arereduced greatly even if the amount of Li is sufficient.

The inventors of the present invention have found that the Li content ofa molten steel can be effectively increased by adding a Si—Li alloyand/or Li₂CO₃ having a Li content between 20 and 40% (% by mass unlessotherwise specified) as a Li-containing substance to the molten steel.

The inventors of the present invention have found that oxide inclusionsoriginating in refractories can be controlled, the Li content of themolten steel contained in a ladle can be increased to a level not lowerthan a predetermined Li content and the effect of Li can be effectivelyexhibited when a Li-containing substance is added to a molten steel (a)after the completion of a series of steps of a ladle refining processincluding composition adjustment, temperature adjustment and slagrefining or (b) at a final stage of a series of steps of a ladlerefining process including composition adjustment, temperatureadjustment and slag refining.

The inventors of the present invention have found that the properties ofan oxide inclusion can be effectively changed by adding a substancecontaining at least one of Ca, Mg, Na and K to the molten steel inaddition to the Li-containing substance.

The present invention has been made on the basis of the foregoingfindings and it is an object of the present invention to provide ahigh-cleanliness steel having a high fatigue strength and high coldworkability and a method of making the same high-cleanliness steel.

(1) A method of making a high-cleanliness steel according to the presentinvention adds a Li—Si alloy and/or Li₂CO₃ having a Li content between20 and 40% (% by mass unless otherwise specified) to a molten steel.

(2) The method of making a high-cleanliness steel according to thepresent invention adds a substance containing at least one of Ca, Mg, Naand K to the molten steel in addition to the Li-containing substance.

(3) The method of making a high-cleanliness steel according to thepresent invention adds the Li-containing substance to the molten steelafter the completion of a series of steps of a ladle refining processincluding composition adjustment, temperature adjustment and slagrefining to control the composition of the molten steel such that themolten steel has a total-Li content between 0.020 and 20 ppm by mass andcontains 1.0 or below oxide inclusion particle having a major diameterof 20 μm or above in 50 g of the steel wire.

(4) The method of making a high-cleanliness steel according to thepresent invention adds the Li-containing substance at a final stage of aseries of steps of a ladle refining process including compositionadjustment, temperature adjustment and slag refining. Thus the steel hasa total-Li content between 0.020 and20 ppm by mass and contains 1.0 orbelow oxide inclusion particle having a major diameter of 20 μm or aboveat a maximum in 50 g of the steel wire. The composition of the oxideinclusion contained in the steel can be adjusted such that the oxideinclusion has a CaO content between 15 and 55%, SiO₂ content between 20and 70%, an Al₂O₃ content of 35% or below, a MgO content of 20% or belowand a Li₂O content between 0.5 and 20%.

The Li-containing substance is added to the molten steel contained in atleast one of a ladle, a tundish (TD) for continuous casting and a mold(MD) for continuous casting. The Li-containing substance is added to themolten steel by an adding means, such as (1) stirring the molten steelwith iron tubular wires containing the Li-containing substance or (2)blowing an inert gas carrying the Li-containing substance into themolten steel.

EFFECT OF THE INVENTION

The method of making a high-cleanliness steel of the present inventionuses a proper Li-containing substance and a proper adding means toadjust the total-Li-content of the steel properly. A high-cleanlinesssteel made by the method of the present invention has excellent coldworkability and fatigue characteristics.

The high-cleanliness steel of the present invention having a total-Licontent between 0.020 and 20 ppm by mass and containing 1.0 or belowoxide oxide inclusion particle having a major diameter of 20 μm or abovein 50 g of the steel wire. Thus the steel has improved cold workabilityand fatigue characteristics.

The oxide inclusion contained in the high-cleanliness steel of thepresent invention is adjusted such that the oxide inclusion has a CaOcontent between 15 and 55%, SiO₂ content between 20 and 70%, an Al₂O₃content of 35% or below, a MgO content of 20% or below and a Li₂Ocontent between 0.5 and 20%. Since the oxide inclusion is soft, has alow melting point and can be easily elongated and broken into pieces,the oxide inclusion will not cause fracture and breakage and the steelhas improved cold workability and fatigue characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relation between the Li-content of and thenumber of inclusion particles contained in a steel in an embodiment forforming steel cords.

FIG. 2 is a graph showing the relation between Li/Si mass ratio and thenumber of inclusion particles in a steel in an embodiment for formingsteel cords.

FIG. 3 is a graph showing the relation between the Li-content of and thesize of the largest inclusion particle in a steel in an embodiment forforming steel cords.

FIG. 4 is a graph showing the relation between Li/Si mass ratio and thesize of the largest inclusion particle in a steel in an embodiment forforming steel cords.

FIG. 5 is a graph showing the relation between the number of oxideinclusion particles contained in a steel and the frequency of breakagein an embodiment for forming steel cords.

FIG. 6 is a graph showing the relation between the size of the largestinclusion particle contained in a steel and the frequency of breakage inan embodiment for forming steel cords.

FIG. 7 is a graph showing the relation between the Li content of and thenumber of oxide inclusion particles in a spring steel in an embodimentfor forming valve springs.

FIG. 8 is a graph showing the relation between Li/Si mass ratio and thenumber of oxide inclusion particles in a spring steel in an embodimentfor forming valve springs.

FIG. 9 is a graph showing the relation between the Li content of and thesize of the largest inclusion particle in a spring steel in anembodiment for forming valve springs.

FIG. 10 is a graph showing the relation between Li/Si mass ratio and thesize of the largest inclusion particle in a spring steel in anembodiment for forming valve springs.

FIG. 11 is a graph showing the relation between the number of oxideinclusion particles in a spring steel and fracture ratio in anembodiment for forming valve springs.

FIG. 12 is a graph showing the relation between the size of the largestinclusion particle in a spring steel and fracture ratio in an embodimentfor forming valve springs.

BEST MODE FOR CARRYING OUT THE INVENTION

High-cleanliness steels in preferred embodiments according to thepresent invention and methods of making those steels use Li effectively.Lithium (Li), differing from other alkaline metals, such as Na and K, iscapable of remarkably changing the properties of a multicomponent oxideinclusion, such as a CaO—Al₂O₃—SiO₂—MnO—MgO multicomponent oxide. In asteel making process, Li combines with a multicomponent oxide andproduces a single-phase multicomponent oxide, such as aCaO—Al₂O₃—SiO₂—MnO—MgO—Li₂O multicomponent oxide. When the steel isheated at a temperature for hot working, phase separation proceeds inthe Li-containing multicomponent oxide inclusion and a vitreous phaseand a crystalline phase develop in the Li-containing multicomponentoxide inclusion. Fine particles of the crystalline phase, namely, anequilibrium phase, deposit in the vitreous inclusion. When this steel isprocessed by blooming or hot rolling, the vitreous inclusion is highlyductile and is easily elongated because the vitreous inclusion has a lowmelting point and a low viscosity. On the other hand, stress isconcentrated on the interface between the crystalline phase and thevitreous phase when the steel is rolled and the crystalline phase andthe vitreous phase are easily separated. Consequently, the inclusionparticles are deformed.

Moreover, Li, which is a strong deoxidizing element, is effective inreducing dissolved oxygen contained in the steel and reducing thequantity of the oxide. When the molten steel contains Li, the productionof detrimental oxides of a SiO₂ system can be suppressed when the moltensteel solidifies.

To make Li exercise its function effectively, Li must be efficientlyadded to the molten steel. A Li-containing substance, such as Li—Sialloy or Li₂CO₃, having a Li content between 20 and 40% is added to themolten steel.

The Li content of the Li—Si alloy is adjusted to a value between 20 and40% to lower the liquidus temperature of the Li—Si alloy during themanufacture of the Li—Si alloy. The low liquidus temperature reduces theevaporation of Li during the manufacture of the Li—Si alloy andincreases the yield of Li. Since the Li—Si alloy of the foregoingcomposition contains a Li—Si intermetallic compound, the yield of Li inthe molten steel can be increased. Preferably, the Li content of theLi—Si alloy is between 25 and 35%. Lithium carbonate (Li₂CO₃) is usedbecause Li₂CO₃ increases the yield of Li.

Preferably, an additional substance containing at least one of Ca, Mg,Na and K is added to the molten steel in addition to the Li-containingsubstance. The additional substance facilitates the combination of Liwith the inclusion. However, the composition of the inclusion differsfrom that of a desired multicomponent oxide if an excessively largeamount of those element is added to the molten steel. Therefore, the Ca,Mg, Na or K content of the molten steel must be 50 ppm at the highest.At least one of those elements may be added to the molten steel eitherbefore or after the addition of the Li-containing substance to themolten steel. Preferably, at least one of those elements is added to themolten steel together with the Li-containing substance when theLi-containing substance is added to the molten steel during a ladlerefining process or at least one of those element is added to the moltensteel before the Li-containing substance is added to the molten steelwhen the Li-containing substance is added to the molten steel after thecompletion of the ladle refining process.

The Li—Si alloy can be produced by premelting. When necessary, Ca, Mg oranother alkaline metal, such as Na or K, may be added to the Li—Sialloy. A diluent metal, such as Fe, may be premixed with the Li—Sialloy. Moreover, Ca, Mg or an alkaline metal, such as Na or K may beadded to lithium carbonate. Since the function of Li is far moreexcellent than those of other alkaline metals, inclusion can besatisfactorily controlled and the cold workability and fatigue strengthof the steel can be satisfactorily improved even if any other alkalinemetal is not added to the Li—Si alloy by premelting or mixing.

A high-cleanliness steel made by the method of the present invention hasthe following properties.

(1) The steel has a total-Li content between 0.020 and 20 ppm by massand contains 1.0 or below oxide inclusion particle having a majordiameter of 20 μm or above in 50 g of the steel wire.

(2) The oxide inclusion contained in the steel has a composition havinga CaO content between 15 and 55%, SiO₂ content between 20 and 70%, anAl₂O₃ content of 35% or below, a MgO content of 20% or below and a Li₂Ocontent between 0.5 and 20%.

The Li-containing substance must be added to the molten steel after thecompletion of a series of steps of a ladle refining process includingthose for composition adjustment, temperature adjustment and slagrefining to control the properties of the steel so as to meet conditionsmentioned in (1). Since the yield of Li is high, the amount of Licontained in the steel is not lower than a predetermined amount afterthe completion of the ladle refining process. Since Li is not added tothe molten steel (the slag) during the ladle refining process, theincrease of inclusions originating in refractories can be prevented.

The Li-containing substance must be added to the molten steel in a finalstage of the series of steps of a ladle refining process includingcomposition adjustment, temperature adjustment and slag refining tocontrol the composition of the inclusion so as to meet conditionsmentioned in (2). The term “a final stage of the ladle refiningprocesses” signifies a period in the second half of a time needed toachieve the series of steps of a ladle refining process including thosefor composition adjustment, temperature adjustment and slag refining.Suppose that 90 min is necessary to complete the series of steps of aladle refining process including those for composition adjustment,temperature adjustment and slag refining. Then, the final stage is aperiod in the second 45 min. It is recommended that the final stage is aperiod in the last ⅓ of the total time; for example, the final stage isthe last 30 min when the total time is 90 min. A period preceding thefinal stage of the series of steps of a ladle refining process is aninitial stage of the ladle refining processes.

Although Li is mixed in the molten steel when the Li-containingsubstance is added to the molten steel in the initial stage of theseries of steps of a ladle refining process, the stirred molten steeltends to contain inclusions originating in refractories. Properties ofsome of those inclusions are not changed by Li and the inclusions remainin hard inclusions.

To properly control Li contained in the high-cleanliness steel havingthe properties mentioned in (1), indices of Li deeply related with thefineness of the inclusions must be controlled. The total-Li content ofthe steel mentioned in (1) and the ratio of the total-Li content to theSi content mentioned in (2), namely, total-Li/Si mass ratio can be theindices. Those indices may be individually used or may be used incombination. The total-Li/Si mass ratio is the ratio of the amount of Lito that of Si contained in the oxide the properties of which are subjectto change. The Li/Si mass ratio is particularly effective in controllingthe properties of Si-deoxidized steels. The total-Li content can bewidely used for controlling the properties of steels other thanSi-deoxidized steels.

It is recommended that the Li content of the steel is 0.020 ppm by massor above, preferably, 0.03 ppm by mass or above, more preferably, 0.1ppm by mass or above to make the function of Li effective. For example,the Li content may be 0.5 ppm by mass or above (for example, 1 ppm bymass or above).

The total-Li/Si mass ratio is 1×10⁻⁶ or above, preferably, 10×10⁻⁶ orabove, more preferably, 50×10⁻⁶ or above. For example, the Li/Si massratio may be 100×10⁻⁶ or above or may be 200×10⁻⁶ or above.

If the steel has an excessively high total-Li content, the number ofparticles of an oxide inclusion (hard inclusion) increases and thenumber of large inclusion particles increases, the cold workability ofthe steel deteriorates and the fatigue strength of the steel decreases.Therefore, it is desirable that the total-Li content of the steel is 20ppm or below by mass, preferably, 9 ppm by mass or below, morepreferably, 6 ppm or below. The Li/Si ratio be 1000×10⁻⁶ or below,preferably, 800×10⁻⁶ or below, more preferably, 600×10⁻⁶ or below.

The high-cleanliness steel made by the method of the present inventionsuppresses increase in inclusions originating in refractories. Forexample, the molten steel contains 1.0 or below oxide inclusion particlehaving a major diameter of 20 μm or above at a maximum in 50 g of thesteel wire, preferably, 0.8 or below oxide inclusion particles in 50 gof the steel wire, more preferably, 0.5 or below oxide inclusionparticles in 50 g of the steel wire.

The cold workability, namely, drawability, and the fatiguecharacteristics of the steel can be improved by controlling the Licontent while the inclusions originating in refractories are thussuppressed, and reducing the size of the inclusion particles.

The composition of the oxide inclusion of the foregoing propertiescontained in the high-cleanliness steel meeting conditions mentioned in(2) can be controlled such that the oxide inclusion is soft, has a lowmelting point and is easy to elongate by hot rolling. The oxideinclusion can be deformed. Thus a high-cleanliness steel excellent incold workability and fatigue characteristics can be produced by reducingthe coarse hard inclusion particles that cause fatigue fracture andbreakage to the least possible extent.

It is generally known that oxides including SiO₂, Al₂O₃, CaO and MgO andmulticomponent oxides contained in steels as oxide inclusions areprincipal substances that induce fatigue fracture and breakage of wiresduring wire drawing. Various techniques including those proposed in theforegoing references that improve the fatigue characteristics of steelsby changing the composition of those oxide inclusions have beenproposed. However, improving methods derived from those known propertymodifying techniques are unable to meet recent user's requirements. Theinventors of the present invention made studies of various additives tomodify the properties of the oxide inclusions by adding a substance tothe steel instead of trying to changing the composition of the oxideinclusions inevitably contained in the steel.

The inventors found through the studies that the ductility of the oxideinclusion contained in the steel can be enhanced beyond the originalductility of the oxide inclusion by effectively utilizing SiO₂, Al₂O₃,CaO and MgO almost inevitably contained in the steel and adding a properamount of Li to the steel and that the highly ductile oxide inclusioncan be easily elongated and deformed by hot rolling, fine particles ofthe oxide inclusion are uniformly dispersed in the hot rolled steel andthereby the fatigue characteristics and drawability of the steel can beremarkably improved. The present invention has been made on the basis ofsuch findings.

The present invention will be described principally in terms of reasonsfor determining oxide contents, i.e., percentages of oxides forming theoxide inclusion.

CaO Content: 15 to 55%

Calcium oxide (CaO) is an essential substance for softening the oxideinclusion so that the oxide inclusion may be easily deformed into smallparticles during a hot rolling process. The oxide inclusion having a lowCaO content is a hard inclusion having a high SiO₂ content or a hardinclusion of a SiO₂—Al₂O₃ system. Such a hard inclusion is difficult todeform by hot rolling and is a principal substance that causes thedeterioration of the fatigue characteristics and drawability of thesteel. Therefore, the CaO content must be 15% or above. It is desirablethat the CaO content is 20% or above, preferably 25% or above. If theCaO content of the oxide inclusion is excessively high, the oxideinclusion has a low hot deformability, becomes a hard, high-CaOinclusion and tends to cause fracture. Therefore, it is desirable thatthe CaO content is 50% or below, preferably, 45% or below.

SiO₂ Content: 20 to 70%

Silicon dioxide (SiO₂), as well as CaO and Al₂O₃, is an essentialsubstance for producing a soft oxide inclusion having a low meltingpoint. The oxide inclusion having a low SiO₂ content below 20% is a hardinclusion consisting principally of CaO and Al₂O₃ in large particles.Such a hard and large inclusion causes fracture. Therefore, the SiO₂content must be 20% or above, preferably, 30% or above. If the SiO₂content of the oxide inclusion is excessively high, the oxide inclusionis a hard inclusion principally consisting of SiO₂ and having a highmelting point, and the tendency of the oxide inclusion to cause breakageand fracture increases. Such a tendency becomes very conspicuous whenthe SiO₂ content is 70% or above. Therefore, it is very important tolimit the SiO₂ content to 70% or below. It is desirable that the SiO₂content is 65% or below, preferably, below 45%, more preferably, 40% orbelow.

Al₂O₃ Content: 35% or Below

Aluminum oxide (Al₂O₃) is not an essential substance for producing asoft oxide inclusion. When the composition of the oxide inclusion isadjusted properly and the oxide inclusion has a proper SiO₂ content; aproper Na₂O content and a proper K₂O content, the oxide inclusion doesnot necessarily contain Al₂O₃. However the oxide inclusion having aproper Al₂O₃ content is soft and has a low melting point. Therefore, itis desirable that the Al₂O₃ content is 5% or above, preferably, 10% orabove. If the Al₂O₃ content of the oxide inclusion is excessively high,the oxide inclusion is a hard inclusion of alumina system difficult todeform by hot rolling and cause fracture and breakage. Therefore, theAl₂O₃ content must be 35% or below, preferably, about 30% or below.

MgO Content: 20% or Below

Magnesium oxide (MgO) is a source of hard inclusion of a MgO—SiO₂ systemand often causes fracture and breakage. Such a trouble occurs frequentlywhen the MgO content is above 20%. Therefore, it is desirable that theMgO content is 20% or below, preferably, 15% or below.

Li₂O Content: 0.5 to 20%

Lithium oxide (Li₂O) is the most specific and the most importantcomponent of the oxide inclusion dealt with by the present invention.Lithium oxide (Li₂O) lowers the melting point and viscosity of amulticomponent oxide inclusion produced in the molten steel andexercises a very important function. To promote the deformation of theoxide inclusion by lowering the melting point and viscosity of the oxideinclusion and to achieve a fatigue characteristic improving effect of alevel intended by the present invention, a desirable Li₂O content is0.5% or above, preferably, 1% or above, more preferably, 2% or above. ALi₂O content exceeding 20% lowers the melting point of the oxideinclusion excessively. Consequently, solubility in refractoriesincreases remarkably, the amount of a hard inclusion originating inlining refractories increases and the fatigue characteristic and coldworkability of the steel are deteriorated. Therefore the Li₂O content ofthe oxide inclusion must be 20% or below, preferably, 15% or below.

Preferably, the oxide inclusion contains Na₂O and/or K₂O in addition toLi₂O. Na₂O and K₂O, similarly to Li₂O, are substances effective inlowering the melting point and viscosity of the multicomponent oxideinclusion. Functions of Na₂O and K₂O can be more effectively exercisedwhen Na₂O and K₂O are used in combination with Li₂O. This point will beexplained in more detail below.

As mentioned above, Li₂O, Na₂O and K₂O have very important functions tolower the melting point and viscosity of the multicomponent oxideinclusion produced in the molten steel and to deform the multi componentoxide inclusion into fine particles. Those oxides are not equivalent infunction. The foregoing effect can be enhanced by making the oxideinclusions contain a proper amount of Li₂O by positively adding Lihaving a strong deoxidizing effect as an oxide inclusion producingsource to the molten steel. The inventors of the present invention foundthrough experiments that Li₂O has a function to make a vitreous oxideinclusion easily crystallizable and that this function is effective inpromoting the deformation of the oxide inclusion and has a conspicuouseffect on the improvement of fatigue characteristics. When a properamount of Li₂O is added to the oxide inclusion having the foregoingcomposition, the oxide inclusion becomes easily crystallizable and finecrystalline particles precipitates in the vitreous oxide inclusion.Consequently, load applied to the oxide inclusion during hot rolling isconcentrated on boundaries each between a vitreous particle and acrystalline particle to promote the fragmentation of the oxideinclusion. Thus the hot rolled steel contains the oxide inclusion infurther deformed particles. Although Li₂O, Na₂O and K₂O haveconsiderable effect even if Li₂O, Na₂O and K₂O are added individually tothe molten steel, addition of Li₂O to the molten steel containing Na₂Oand K₂O produces a high favorable synergistic effect on the improvementof fatigue characteristics and drawability of the steel.

Moreover, Li having a strong deoxidizing function contributes to thereduction of dissolved oxygen contained in the steel. Thus Li suppressesthe production of an inclusion having a high SiO₂ content thatprecipitates when the molten steel solidifies and the growth ofprecipitated particles. When the molten steel solidifies, Li, Na and Kcontained in the molten steel produce multicomponent oxides ofSiO₂—Li₂O, SiO₂—Na₂O and SiO₂—K₂O systems and a mixture of thosemulticomponent oxides to suppress the formation of the inclusion havinga high SiO₂ content.

It is recommended that Li₂O/SiO₂ mass ratio, namely, the ratio in massof Li₂O contained in the oxide inclusion to SiO₂ contained in the oxideinclusion, is within a predetermined range when Li is essential. Li₂O isimportant to lowering the melting point and viscosity of themulticomponent oxide and to promoting the deformation of themulticomponent oxide inclusion. The ratio in mass of Li₂O to SiO₂ thatforms a network is important. The effect of Li₂O on lowering the meltingpoint and viscosity of the multicomponent oxide inclusion multiplieswhen the Li₂O/SiO₂ mass ratio is sufficiently high. Thus the inclusioncan be further deformed and the fracture of the steel starting fromlarge SiO₂ particles can be surely prevented. If the Li₂O/SiO₂ massratio is excessively high, the melting point and viscosity of themulticomponent oxide inclusion are lowered, refractories are melted,hard inclusions originating in refractories increase, and the fatiguecharacteristics and cold workability of the steel are deteriorated. Inview of the foregoing facts, it is desirable, when Li is essential, thatthe Li₂O/SiO₂ mass ratio is between, for example, about 0.01(preferably, about 0.02 or above, more preferably, about 0.03 or above)and about 0.5 or below (preferably, 0.4 or below).

According to the present invention, MnO, namely, another oxide, happento be added in the oxide inclusion. MnO is scarcely effective in causingfatigue fracture and breakage and is reduced by a strong deoxidizer,such as Ca, Al or Li. Therefore, there is not any particular limit tothe MnO content of the oxide inclusion.

The high-cleanliness steel made by the method of the present inventionis excellent in cold workability and fatigue characteristics. Therefore,the high-cleanliness steel can be advantageously used for forminghigh-tension steel wires, fine steel wires and high-strength springs,such as valve springs. A high-cleanliness steel made by the steel makingmethod intended for such uses has a C content of 1.2% or below(preferably, between 0.1 and 1.0%, more preferably, between 0.3 and0.9%), a Si content between 0.1 and 4% (preferably, between 0.1 and 3%,more preferably, 0.2 and 2.5%), a Mn content between 0.1 and 2%(preferably, 0.2 and 1.5%, more preferably, between 0.3 and 1.2%), atotal Al content (percentage of the sum of an amount of Al contained inthe steel and an amount of Al contained in the inclusion) of 0.01% orbelow (preferably, 0.008% or below, more preferably, 0.005% or below)and an O content of 0.005% or below (preferably, 0.004% or below, morepreferably, 0.003% or below) The desirable C content of 1.2% or below isdetermined with an intention to use the high-cleanliness steel of thepresent invention for forming high-strength steel wires (C content: onthe order of 1.1%) and very fine steel wires (C content: on the order of0.01%). High-carbon steels having a C content exceeding 1.2% areexcessively hard, have low workability and are practically difficult touse.

The present invention uses Li to modify the multicomponent oxideinclusion of the CaO—Al₂O₃—SiO₂—MnO—MgO system. In most cases, Ca and Mgcontained in the multicomponent oxide inclusion are combined with themolten steel during ladle refining process through the inclusion of topslag. When necessary, Ca and Mg may be added to the molten steel. Insome cases, a SiO₂-rich or Al₂O₃-rich secondary deoxidation product isproduced when the molten steel solidifies and such a secondarydeoxidation product causes troubles. In some cases, addition of Ca, Mgand Li to the molten steel is effective in preventing such troubles. Thesecondary deoxidation product develops from a primary inclusion ordevelops individually. In some cases, the inclusion generated bysecondary deoxidation tend to become SiO₂-rich or Al₂O₃-rich as comparedwith that included in the molten steel of the tundish or the like. WhenCa, Mg and Li are added to the molten steel, a multicomponent oxideinclusion as a secondary deoxidation product containing SiO₂, Al₂O₃,CaO, MgO and Li₂O is produced and the production of a SiO₂-rich orAl₂O₃-rich secondary deoxidation product can be suppressed.

A desirable total Ca content (percentage of the sum of an amount of Cacontained in the steel and an amount of Ca contained in the inclusion)is between 0.1 and 40 ppm by mass, preferably, between about 0.2 andabout 25 ppm by mass), a desirable total Mg content (percentage of thesum of an amount of Mg contained in the steel and an amount of Mgcontained in the inclusion) is between about 0.1 and 15 ppm by mass,preferably, between about 0.2 and about 10 ppm by mass.

When necessary, the steel may contain property improving elements, suchas Cr, Ni, V, Nb, Mo, W, Cu and Ti. The steel may contain one or some ofthose property improving elements. Preferably, the steel has a Crcontent of 3% or below, preferably, between 0.01 and 1%, a Ni content of1% or below, preferably, between 0.05 and 0.5%, a V content of 0.5% orbelow, preferably, between 0.005 and 0.2%, a Nb content of 0.1% orbelow, preferably, between 0.005 and 0.05%, a Mo content of 1% or below,preferably, between 0.01 and 0.5%, a W content of 1% or below,preferably, between 0.01 and 0.5%, a Cu content of 2% or below,preferably, between 0.05 and 1%, a Ti content of about 0.06% or below,preferably, between 0.005 and 0.03%. Elements other than those elementsmay be Fe and unavoidable impurities.

A high-cleanliness steel most suitable for forming high-strength finesteel wires and high-strength valve springs contains C, Si and Mn. Ahigh-cleanliness steel most suitable for forming, for example,high-strength fine steel wires has a C content between 0.5 and 1.2%,preferably, between 0.7 and 1.1%, a Si content between 0.1 and 0.5%,preferably, 0.15 and 0.4% and a Mn content between 0.2 and 1%,preferably, between 0.3 and 0.8%. A high-cleanliness steel most suitablefor forming high-strength valve springs has a C content between 0.3 and1.0%, preferably, between 0.4 and 0.8%, a Si content between 1 and 4%,preferably, 1.2 and 2.5% and a Mn content between 0.3 and 1.5%,preferably, between 0.4 and 1.0%.

EXAMPLE 1

The present invention will be morespecifically described in terms ofexamples. It is to be noted that the following are only examples, thepresent invention is not limited by the following examples, variouschanges and variations are possible therein without departing from theteaching and scope of the present invention.

Experiment 1

An experiment using actual machines (or laboratory machines) wasconducted. In the experiment using actual machines, a molten steelcontained in a converter was poured into a ladle (500 kg of a steelsimilar to that produced by a converter was made in the laboratory),various fluxes were added to the molten steel, and the molten steel wassubjected to composition adjustment, electrode heating and Ar bubblingfor a ladle refining process (slag refining process). When necessary, Caand Mg were added to the molten steel. Li was added to the molten steelin Li₂O, Li₂CO₃, Li—Si alloy or LiF before the ladle refining process,at an initial stage of the ladle refining process or after the ladlerefining process. Li was added to the molten steel in the ladle, in thetundish (TD) for continuous casting or in a mold (MD) for continuouscasting in various adding modes respectively using tubular wires, aninjection device and a dropping device. Then, blooms or ingots were madeby casting the molten steel. A mold used in the laboratory experiment isequivalent in cooling rate with an actual mold. Steel wires of 5.5 mm indiameter were made by subjecting the blooms or the ingots to a bloomrolling process, a forging process and a hot rolling process. Steels ofa composition suitable for forming springs and steels of a compositionsuitable for forming steel cords were made.

The steels were evaluated in terms of the Li content of the steel wires,the inclusion morphology and the composition of the inclusion determinedthrough the microscopic observation of the L sections of the steelwires. The steel wires were dissolved in an acid solution to count thenumber and to measure the size of hard inclusion particles. The steelwires for forming springs were subjected to a rotary bending fatiguetest. The steel wires for forming steel cords were subjected to wiredrawing test.

Li Content of Steel

A 0.5 g of a test sample was sampled from the steel wire and the testsample was heated for hot decomposition in a mixed acid solutionprepared by mixing H₂O, HCl and HNO₃ and contained in a beaker to obtaina test solution. The test solution was cooled by natural cooling. Thecooled test solution was poured into a separating funnel, HCl was addedto the test solution to adjust the acidity of the test solution to 9N.Methylisobutyl ketone (MIBK) was added to the test solution, the beakercontaining the test solution was shook to extract iron in a MIBK phase.MIBK was added again to the test solution after removing the MIBK phase.This extracting and separating cycle was repeated three times toseparate iron completely from the test solution. The 9N-hydrochloricacid acidic phase was diluted to obtain 100 ml of an alkaline testsolution.

The Li (mass number 7) concentration of the alkaline test solution wasmeasured by an ICP mass analyzer (SPQ8000, Seiko Instruments) and the Licontent of the steel was calculated by using the measured Liconcentration. Conditions for ICP mass analysis were as follows.

High-frequency power 1.2 kW

Flow rate of carrier gas: 0.4 l/min

Number of Oxide Inclusion Particles of 20 μm or Above in Major Diameter

Fifteen hundred grams of the test steel wire was cut into about 100 g ofsteel chips and the steel chips were scaled. The scaled steel chips weredissolved in a nitric acid solution of about 90° C. to obtain a testsolution. The test solution was filtered by a filter having 10 μmmeshes. Inclusion particles filtered out by the filter were analyzed byEPMA (electron probe microanalysis). Major diameters of oxide inclusionparticles (hard inclusion particles) were measured. Large oxideinclusion particles having major diameters of 20 μm or above werecounted. The number of the large oxide inclusion particles per 50 g ofthe steel was calculated.

Wire Drawing Test (Frequency of Breakage)

The 5.5 mm diameter steel wire formed by hot rolling was drawn into a2.5 mm diameter steel wire by a primary drawing process. The 2.5 mmdiameter steel wire was processed by a heat treatment (air patentingprocess). Then, the 2.5 mm diameter steel wire was drawn into a 0.8 mmdiameter steel wire by a secondary drawing process. The 0.8 mm diametersteel wire was subjected to a heat treatment (lead patenting process)and was plated with brass. The brass-plated 0.8 mm diameter steel wirewas drawn into a 0.15 mm diameter steel wire by a wet drawing process.The frequency of breakage of the 0.15 mm diameter wire during the wetdrawing process was counted and the frequency of breakage was convertedinto a frequency of breakage per 10 t of the 0.15 mm diameter steelwires.

Fatigue Strength Test (Fracture Ratio)

The 5.5 mm diameter steel wire formed by hot rolling was subjectedsequentially to a shaving process (SV), a low-temperature annealingprocess (LA), a cold drawing process (diameter: 4.0 mm), an oiltempering process (continuous tempering process for oil quenching andtempering in a lead bath at about 450° C.), a simplified stress reliefannealing process (bluing process at about 400° C.), a shot peeningprocess and a stress relief annealing process to obtain test steel wiresof 4.0 mm in diameter and 650 mm in length. The fatigue strength of thetest steel wires was measured by a Nakamura type rotating bendingfatigue tester. Fatigue test conditions were: 940 MPa in nominal stress,4000 to 5000 rpm in rotating speed and 2×10⁷ in the number of bendingcycles. The number of the test steel wires caused to fracture by theinclusions before 2×10⁷ bending cycles was counted and fracture ratiowas calculated by using the following expression.(Fracture ratio) (%)=[(Number of steel wires caused to fracture byinclusion before 2×10⁷ bending cycles)/{{Number of steel wires caused tofracture by inclusion before 2×10⁷ bending cycles}+(Number of steelwires not fractured after 2×10⁷ bending cycles)}]×100

Size of the Largest Inclusion Particle

Sections of the test steel wires caused to fracture by the inclusionduring the drawing test and the fatigue strength test were observed bymeans of a SEM. The width of the largest inclusion particle in each ofthe section was measured and the composition of the largest inclusionparticle was analyzed by EPMA.

Results of the foregoing tests of the test steel wires obtained byExperiment 1 are shown in Tables 1 and 2. Table 1 shows results of thewire drawing test of the test steel wires obtained by Experiment 1simulating steel cords and Table 2 shows results of the fatigue strengthtest of the test steel wires obtained by Experiment 1 simulating valvesprings.

TABLE 1 Number of oxide Size of Li content of steel/ inclusion particlesof the largest Composition Li adding method Si content of Li₂O contentof 20 μm or above Drawability inclusion Sample C Si Mn Al Ca Mg O LiLi-containing Adding Adding steel slag (Number per 50 g of (Frequencyparticle No. (% by mass) (ppm) material Adding time position method (%by mass) (% by mass) steel) of breakage) (μm) A1 0.72 0.21 0.52 0.00310.1 0.7 10 0.030 Li₂CO₃ After the steel Ladle Wire 1.43 × 10⁻⁵ 0.3 0.205 20 melting process A2 0.70 0.20 0.52 0.004 13.1 1.0 13 0.105 Li₂CO₃ +Ca After the steel Ladle Wire 5.25 × 10⁻⁵ 0.2 0.18 6 18 melting processA3 0.73 0.20 0.49 0.003 13.0 1.2 15 0.240 Li₂CO₃ After the steel LadleWire 1.20 × 10⁻⁴ 0.1 0.21 6 18 melting process A4 0.72 0.18 0.49 0.00312.5 0.8 14 0.320 Li₂CO₃ + After the steel Ladle Wire 1.78 × 10⁻⁴ 0.20.10 2 17 Ca, Na, K melting process A5 0.83 0.21 0.52 0.004 14.5 0.8 160.460 Li₂CO₃ After the steel Ladle Injection 2.19 × 10⁻⁴ 0.1 0.09 0 15melting process A6 0.83 0.20 0.50 0.003 14.1 2.5 15 0.665 Li₂CO₃ + Ca,After the steel TD Injection 3.35 × 10⁻⁴ — 0.23 5 16 Mg melting processA7 0.82 0.22 0.50 0.003 15.1 0.9 15 0.520 Li₂CO₃ + Ca After the steel MDWire 3.36 × 10⁻⁴ — 0.18 1 16 melting process A8 0.82 0.22 0.51 0.00312.1 1.1 14 0.870 Li-70% Si After the steel Ladle Injection 3.95 × 10⁻⁴0.3 0.16 3 18 alloy melting process A9 0.81 0.20 0.50 0.004 15.8 2.3 151.210 Li-75% Si After the steel TD Wire 6.05 × 10⁻⁴ 0.5 0.13 5 15 alloymelting process A10 0.83 0.21 0.52 0.003 14.1 2.7 14 1.750 Li-70% SiAfter the steel Ladle Wire 8.33 × 10⁻⁴ 0.5 0.25 8 18 alloy + Ca, Mgmelting process A11 0.83 0.22 0.50 0.003 15.0 2.5 15 1.970 Li-60% SiAfter the steel Ladle Wire 8.95 × 10⁻⁴ 0.6 0.19 6 20 alloy meltingprocess A12 0.72 0.21 0.52 0.003 23.8 1.8 24 13.200 Li-70% Si Initialstage of the Ladle Wire 6.29 × 10⁻³ 1.7 1.02 48 45 alloy steel meltingprocess A13 0.73 0.22 0.52 0.003 19.8 1.4 21 11.200 Li-70% Si Initialstage of the Ladle Wire 5.09 × 10⁻³ 2.6 1.02 37 35 alloy steel meltingprocess A14 0.83 0.22 0.51 0.003 24.3 1.7 25 20.200 Li-70% Si Initialstage of the Ladle Wire 9.18 × 10⁻³ 4.1 1.20 60 41 alloy steel meltingprocess A15 0.83 0.20 0.51 0.004 22.9 3.7 25 0.018 Li-70% Si Initialstage of the Ladle Input 9.00 × 10⁻⁶ 5.2 1.10 51 39 alloy steel meltingprocess A16 0.81 0.25 0.50 0.003 9.6 0.7 10 10 Li₂O- Initial stage ofthe Ladle Alloy input 0.004 10.0 2.50 87 52 containing slag steelmelting process A17 0.82 0.25 0.50 0.003 14.5 1.4 18 0.017 2% Li-90%Initial stage of the Ladle Alloy input 6.80 × 10⁻⁶ 1.5 1.10 54 42Si-0.5% Al steel melting process A18 0.82 0.25 0.50 0.003 12.1 1.1 170.018 5% Li-89% Initial stage of the Ladle Input 7.20 × 10⁻⁶ 1.0 1.05 5139 Si-0.1% Al steel melting process A19 0.73 0.23 0.52 0.003 10.4 1.5 210.008 LiF Initial stage of the Ladle Input 3.48 × 10⁻⁶ 1.2 1.20 56 34steel melting process A20 0.72 0.20 0.52 0.004 9.2 1.1 17 0.011 LiFInitial stage of the Ladle Input 5.50 × 10⁻⁶ 1.6 1.29 54 32 steelmelting process A21 0.81 0.19 0.51 0.004 8.5 1.0 16 0.014 LiF Initialstage of the Ladle Input 7.37 × 10⁻⁶ 1.7 1.14 49 32 steel meltingprocess A22 0.82 0.21 0.51 0.003 10.1 1.7 22 0.005 LiF Initial stage ofthe Ladle Input 2.38 × 10⁻⁶ 0.0 1.26 55 32 steel melting process A230.82 0.25 0.50 0.003 9.9 1.6 20 0.018 Na₂SiO₃ + LIF Before the steelLadle Input 7.20 × 10⁻⁶ 4.1 2.01 60 41 melting process

Data shown in Table 1 are shown in graphs in FIGS. 1 to 6.

TABLE 2 Li content Fatigue Size of of steel/ strength the largestComposition Alkaline metal adding method Si content of Li₂O content(Fracture Fracture inclusion Sample C Si Mn Al Cr N V Ca Mg O LiLi-containing Adding Adding steel of slag ratio) causing particle No. (%by mass) (ppm) material Adding time position method (% by mass) (% bymass) (%) inclusion (μm) B1 0.55 1.45 0.71 0.004 0.70 — — 10.2 1.0 100.020 Li₂CO₃ After the steel Ladle Wire 1.38 × 10⁻⁵ 0.1 4 SiO₂-rich 15melting process B2 0.55 1.46 0.70 0.003 0.71 — — 18.1 1.4 18 0.330Li₂CO₃ + Ca After the steel Ladle Wire 2.26 × 10⁻⁵ 0.1 0 MgO—SiO₂ 12melting process B3 0.55 1.46 0.70 0.003 0.71 — — 22.0 1.8 24 1.190Li₂CO₃ After the steel Ladle Wire 8.15 × 10⁻⁵ 0.1 1 MgO—SiO₂ 15 meltingprocess B4 0.55 1.46 0.70 0.003 0.71 0.25 0.10 17.3 3.3 22 0.624Li2CO3 + After the steel Ladle Wire 4.27 × 10⁻⁵ 0.2 0 — 0 Ca, Mg meltingprocess B5 0.58 1.45 0.70 0.004 0.70 0.25 0.10 16.1 4.3 21 0.986 Li₂CO₃After the steel Ladle Injec- 8.80 × 10⁻⁵ 0.3 0 — 0 melting process tionB6 0.60 1.46 0.70 0.005 0.70 0.25 0.10 20.9 5.1 21 0.815 Li₂CO₃ Afterthe steel Ladle Injec- 5.58 × 10⁻⁵ 0.4 2 Al₂O₃-rich 16 melting processtion B7 0.63 1.45 0.65 0.003 0.65 — 0.09 19.8 1.3 21 0.893 Li₂CO₃ Afterthe steel TD Wire 6.16 × 10⁻³ 0.2 0 — 0 melting process B8 0.65 1.450.65 0.004 0.65 — 0.09 19.0 1.6 20 1.708 Li₂CO₃ After the steel MD Wire1.18 × 10⁻⁴ 0.3 3 CaO—SiO₂ 13 melting process B9 0.63 1.45 0.65 0.0030.65 — 0.09 24.3 7.5 25 5.300 Li-70% Si After the steel Ladle Wire 3.66× 10⁻⁴ 0.3 3 Al₂O₃-rich 15 alloy melting process B10 0.64 1.46 0.640.004 0.65 — 0.09 23.1 4.1 22 13.00 Li-70% Si After the steel Ladle Wire8.90 × 10⁻⁴ 0.3 4 Refractory 21 alloy + Ca melting process base B11 0.602.00 0.89 0.005 0.90 0.25 0.10 17.8 3.2 19 0.470 Li₂CO₃ After the steelLadle Wire 2.35 × 10⁻⁵ 0.2 1 SiO₂-rich 20 melting process B12 0.61 2.010.90 0.005 0.90 0.25 0.10 17.1 2.8 19 3.887 Li-70% Si After the steelLadle Wire 1.93 × 10⁻⁴ 0.1 3 Al₂O₃-rich 16 alloy + Ca, Mg meltingprocess B13 0.61 2.01 0.90 0.005 0.90 0.25 0.10 15.0 1.1 15 0.510 Li₂CO₃After the steel Ladle Wire 2.54 × 10⁻⁵ 0.2 2 MgO—SiO₂ 17 melting processB14 0.61 2.01 0.90 0.005 0.90 0.25 0.10 20.2 1.4 19 2.110 Li-70% SiAfter the steel Ladle Wire 1.05 × 10⁻⁴ 0.3 2 MgO—SiO₂ 14 alloy meltingprocess B15 0.61 2.01 0.90 0.005 0.90 0.25 0.10 21.0 1.0 21 5.90 Li-70%Si After the steel TD Injec- 2.94 × 10⁻⁴ 0.3 3 Refractory 19 alloymelting process tion base B16 0.61 2.01 0.90 0.005 0.90 0.25 0.10 20.71.6 22 8.10 Li-70% Si After the steel Ladle Injec- 4.03 × 10⁻⁴ 0.2 2Refractory 18 alloy melting process tion base B17 0.61 2.01 0.90 0.0050.90 0.25 0.10 17.0 2.7 18 9.20 Li-70% Si After the steel TD Wire 4.58 ×10⁻⁴ 0.1 4 Refractory 19 alloy melting process base B18 0.61 2.01 0.900.005 0.90 0.25 0.10 38.0 8.8 30 19.90 Li-70% Si After the steel MD Wire9.90 × 10⁻⁴ 0.3 5 Refractory 21 alloy melting process base B19 0.58 1.450.71 0.003 0.71 — — 9.1 0.9 21 0.009 LiF Initial stage of the Ladle Wire6.21 × 10⁻⁷ 0.1 35 SiO₂-rich 38 steel melting process B20 0.55 1.46 0.730.004 0.70 — — 23.2 4.0 24 20.20 Li-70% Si Initial stage of the LadleWire 1.38 × 10⁻³ 3.0 46 Refractory 48 alloy steel melting process baseB21 0.58 1.45 0.71 0.003 0.68 0.25 0.10 8.0 0.7 17 0.010 LiF Initialstage of the Ladle Wire 6.90 × 10⁻⁷ 0.2 37 SiO₂-rich 39 steel meltingprocess B22 0.58 1.46 0.71 0.004 0.70 0.25 0.10 19.9 4.0 21 2.140 Li-70%Si Initial stage of the Ladle Injec- 1.47 × 10⁻² 7.0 54 Refractory 48alloy steel melting process tion base B23 0.63 1.45 0.65 0.003 0.65 —0.09 7.2 0.8 16 0.018 Li-95% Si Initial stage of the Ladle Wire 1.24 ×10⁻⁸ 0.3 39 SiO₂-rich 41 alloy steel melting process B24 0.65 1.45 0.650.004 0.65 — 0.09 25.1 4.0 25 21.80 Li-70% Si Initial stage of the LadleWire 1.50 × 10⁻³ 5.0 51 Refractory 60 alloy steel melting process baseB25 0.60 2.00 0.89 0.005 0.90 0.25 0.10 8 0.9 22 0.012 LiF Initial stageof the Ladle Wire 6.00 × 10⁻⁷ 0.2 45 SiO₂-rich 50 steel melting processB26 0.60 2.00 0.89 0.005 0.90 0.25 0.10 20.2 3.0 19 23.00 Li-70% SiInitial stage of the Ladle Wire 1.17 × 10⁻³ 4.0 60 Refractory 53 alloysteel melting process base

Data shown in Table 2 are shown in graphs shown in FIGS. 7 to 12.

As obvious from the test results shown in Tables 1 and 2, when the Li isadded to the steel before or at an initial stage of the ladle refiningprocess the molten steel contains a large amount of inclusionsoriginating in refractories when it is desired that the molten steelcontains a proper amount of Li, (Sample Nos. A12 to A14, A16, B20, B22,B24 and B26) and the molten steel is unable to contain a sufficientamount of Li when it is desired to reduce inclusions originating inrefractories (Sample Nos. A15, A17 to A23, B19, B21, B23 and B25). WhenLi is added to the molten steel after the ladle refining process toincrease the yield of Li, inclusions originating in refractories can bereduced, the steel contains a proper amount of Li and the steel containsLi in a proper Li/Si ratio (Sample Nos. A1 to A11 and B1 to B18).Consequently, the inclusions can be deformed, the number of inclusionparticles of 20 μm or above in major diameter and the size of thelargest inclusion particle can be reduced, and the steel has improveddrawability (frequency of breakage) and improved fatigue strength(fracture ratio).

EXAMPLE 2

Experiment 2

An experiment using actual 90t and 250t machines (or laboratorymachines) was conducted. In the experiment using actual machines, amolten steel contained in a converter was poured into a ladle (500 kg ofa steel similar to that produced by a converter was made in thelaboratory), various fluxes were added to the molten steel, and themolten steel was subjected to composition adjustment, electrode heatingand Ar bubbling for a ladle refining process. In a slag refiningprocess, a Li-70% Si alloy (30% Li-70% Si alloy), Ca—Si wires, and ablend of Li₂CO₃, Na₂CO₃, K₂CO₃, Ca wires and Mg wires were added to themolten steel during the ladle refining process (at an initial or finalstage of the ladle refining process). After the completion of therefining process, blooms or ingots were made by casting the moltensteel. The laboratory experiment used a mold equivalent in cooling ratewith an actual mold. Steel wires of 5.5 mm in diameter were made bysubjecting the billets to a bloom rolling process or a forging processand a hot rolling process. Steels of a composition suitable for formingsprings and steels of a composition suitable for forming steel cordswere made.

The steels were evaluated in terms of inclusions in the inclusionmorphology, and the composition of the inclusion determined through themicroscopic observation of the L sections of the steel wires. The steelwires were dissolved in an acid solution to count the number and tomeasure the size of hard inclusion particles and to examine thecomposition of the hard inclusions. The steel wires for forming springswere subjected to a rotary bending fatigue test. The steel wires forforming steel cords were subjected to wire drawing test.

Inclusions Contained in Steel Wires

The L-section of a 5.5 mm diameter wire of 80 mm in length was polishedand the thickness and length of inclusions were measured, the number ofinclusions was counted and the composition of inclusions was analyzed.

Analysis of Composition of Inclusions

The Li₂O content of the inclusion cannot be determined by EPMA. The Li₂Ocontent of the inclusion was measured by the following procedure of SIMS(secondary ion mass spectrometry)

(1) Primary Standard Sample

1) Synthetic oxides of compositions excluding Li₂O and synthetic oxidesof compositions including Li₂O were made as standard samples. Thosesynthetic oxides were analyzed by chemical quantitative analysis.

2) The Li-to-Si relative secondary ionic strength of each of thesynthetic oxide was measured.

3) Analytical curves of the Li-to-Si relative secondary ionic strengthand the Li₂O content determined by quantitative analysis in 1) weredrawn.

(2) Secondary Standard Sample (Environmental Measurement Correction)

1) A standard sample was made by implanting Li ions in a Si wafer. Thestandard sample is used for environmental measurement correction.Li-to-Si secondary ion strength measured in (1)-2) was corrected byusing the standard sample.

(3) Actual Measurement

1) The inclusion contained in the steel was analyzed by EDX and EPMA todetermine the CaO, MgO, Al₂O₃, MnO, SiO₂, Na₂O and K₂O contents of theinclusion.

2) The Li-to-Si relative secondary ionic strength of each of theinclusion contained in the steel was measured. The analytical curve theclosest to the results of analysis made in (3)-1) is selected from theanalytical curves drawn in (1)-3) to determine Li₂O content.

Data thus obtained are shown in Tables 3 and 4. Table 3 shows results ofthe wire drawing test of the test steel wires obtained by Experiment 2simulating steel cords and Table 4 shows results of the fatigue strengthtest of the test steel wires obtained by Experiment 2 simulating valvesprings.

TABLE 3 Component content (% by mass) Composition of inclusion (% bymass) Sample No. C Si Mn Al CaO SiO₂ MgO Al₂O₃ Na₂O K₂O Li₂O Na₂O +K₂O + Li₂O A24 0.72 0.21 0.52 0.003 39.8 39.3 2.1 18.1 0.0 0.0 0.7 0.7A25 0.82 0.19 0.51 0.004 37.2 39.0 3.7 16.0 0.0 0.0 4.1 4.1 A26 0.810.21 0.50 0.003 20.8 41.9 2.1 15.6 3.4 4.1 12.1 19.6 A27 0.72 0.18 0.490.003 29.0 45.0 18.0 3.0 0.0 1.0 3.0 4.0 A28 0.72 0.21 0.52 0.004 15.047.4 9.2 16.0 2.3 0.0 10.1 12.4 A29 0.83 0.19 0.50 0.003 20.1 63.1 1.88.0 0.5 2.6 3.9 7.0 A30 0.84 0.20 0.48 0.003 26.4 32.0 2.5 34.0 0.0 0.05.1 5.1 A31 0.70 0.18 0.49 0.003 30.0 58.0 1.6 10.0 0.0 0.0 0.4 0.4 A320.73 0.20 0.51 0.003 25.0 39.0 1.0 14.0 0.0 0.0 21.0 21.0 A33 0.73 0.210.50 0.004 52.2 25.0 2.2 16.6 1.0 0.0 0.0 1.0 A34 0.74 0.20 0.50 0.00340.6 61.0 2.5 21.3 0.0 1.2 0.0 1.2 A35 0.81 0.19 0.52 0.003 28.7 31.03.4 36.0 0.0 0.0 0.9 0.9 A36 0.83 0.20 0.49 0.004 11.0 50.7 20.5 16.20.6 0.0 1.0 1.6 A37 0.83 0.20 0.50 0.003 15.0 71.0 3.9 8.1 0.5 0.4 1.12.0 A38 0.83 0.19 0.51 0.004 45.0 18.0 2.6 33.4 0.0 0.4 0.6 1.0 Numberof oxide inclusion particles of 20 Drawability Size of the larg- Liadding method μm or above (Number (Frequency est inclusion Sample No.Li-containing material Adding time per 50 g of steel) of breakage)particle (μm) A24 Li₂CO₃ Final stage of the steel 0.20 10 22 meltingprocess A25 Li-70% Si alloy Final stage of the steel 0.10 5 19 meltingprocess A26 Li-70% Si alloy Final stage of the steel 0.09 2 17 meltingprocess A27 Li-70% Si alloy Final stage of the steel 0.16 6 20 meltingprocess A28 Li-70% Si alloy Final stage of the steel 0.13 7 18 meltingprocess A29 Li-70% Si alloy Final stage of the steel 0.19 11 22 meltingprocess A30 Li-70% Si alloy Final stage of the steel 0.21 9 21 meltingprocess A31 Li₂CO₃ Initial stage of the steel 0.80 31 32 melting processA32 Li-70% Si alloy Initial stage of the steel 0.55 28 41 meltingprocess A33 — — 0.67 35 45 A34 — — 0.45 40 36 A35 Li₂CO₃ Initial stageof the steel 0.38 24 31 melting process A36 Li₂CO₃ Initial stage of thesteel 0.54 46 37 melting process A37 Li₂CO₃ Initial stage of the steel0.93 58 39 melting process A38 Li₂CO₃ Initial stage of the steel 0.41 2232 melting process

TABLE 4 Fracture Inclusion Size of the largest Sample Component content(% by mass) Composition of inclusion (% by mass) Li adding method ratiocausative of inclusion particle No. C Si Mn Al Cr Ni V CaO SiO₂ MgOAl₂O₃ Na₂O K₂O Li₂O Na₂O + K₂O + Li₂O Adding position Adding time (%)fracture (μm) B27 0.55 1.45 0.71 0.004 0.70 — — 35.0 42.7 2.9 18.8 0.00.0 0.6 0.6 Li₂CO₃ Final stage of the steel 18 MgO—SiO₂ 25 meltingprocess B28 0.50 1.48 0.70 0.003 0.71 0.25 0.10 35.2 40.5 2.7 18.0 0.01.0 2.6 3.6 Li₂CO₃ Final stage of the steel 0 — — melting process B290.58 1.45 0.74 0.004 0.70 0.25 0.10 25.6 41.5 17.3 10.2 1.2 0.0 4.2 5.4Li₂CO₃ Final stage of the steel 0 — — melting process B30 0.60 1.40 0.700.040 0.70 0.25 0.10 22.4 37.8 2.4 33.8 0.0 0.0 3.8 3.6 Li₂CO₃ Finalstage of the steel 8 Al₂O₃-rich 16 melting process B31 0.63 1.45 0.650.003 0.65 — 0.09 21.2 40.2 2.5 28.6 2.1 1.5 3.9 7.5 Li₂CO₃ Final stageof the steel 0 — 0 melting process B32 0.65 1.45 0.65 0.064 0.05 — 0.0936.3 46.0 5.6 4.9 0.0 0.0 7.2 7.2 Li-70% Si alloy Final stage of thesteel 3 CaO—SiO₂ 13 melting process B33 0.60 2.06 0.00 0.005 0.00 0.250.10 18.0 69.8 2.8 7.2 0.0 0.0 2.2 2.2 Li₂CO₃ Final stage of the steel18 SiO₂-rich 25 melting process B34 0.60 2.01 0.00 0.005 0.00 0.25 0.1015.1 45.0 2.2 19.5 0.0 0.0 18.2 18.2 Li-70% Si alloy Final stage of thesteel 5 Al₂O₃-rich 18 melting process B35 0.55 1.44 0.69 0.003 0.70 — —21.5 60.3 2.9 13.2 0.0 0.0 2.1 2.1 Li₂CO₃ Final stage of the steel 0MgO—SiO₂ 18 melting process B36 0.56 1.46 0.70 0.003 0.71 0.25 0.10 20.365.2 2.4 10.2 0.0 0.0 1.9 1.9 Li₂CO₃ Final stage of the steel 3 MgO—SiO₂19 melting process B37 0.63 1.45 0.65 0.003 0.65 — 0.08 20.2 63.8 2.97.2 0.0 0.0 5.9 5.9 Li-70% Si alloy Final stage of the steel 0 MgO—SiO₂16 melting process B38 0.00 2.00 0.69 0.005 6.90 0.25 0.10 19.0 62.5 2.89.2 0.0 0.0 6.5 6.5 Li-70% Si alloy Final stage of the steel 0 MgO—SiO₂0 melting process B39 0.50 1.45 0.71 0.003 0.71 — — 19.2 64.8 2.4 13.20.0 0.0 0.4 0.4 Li₂CO₃ Initial stage of the 35 SiO₂-rich 32 steelmelting process B40 0.55 1.40 0.73 0.004 0.70 — — 20.9 43.1 2.5 12.3 0.00.0 21.2 21.2 Li-70% Si alloy Initial stage of the 75 Refractory 48steel melting process base B41 0.55 1.40 0.68 0.005 0.70 — — 15.0 45.02.3 16.2 2.5 1.2 17.8 17.8 Li-70% Si alloy Initial stage of the 89Refractory 67 steel melting process base B42 0.50 1.45 0.71 0.003 6.600.25 0.1 55.9 32.6 2.1 8.4 0.0 0.0 1.0 1.0 Li₂CO₃ Initial stage of the35 CaO—SiO₂ 35 steel melting process B43 0.50 1.40 0.71 0.004 0.70 0.250.1 14.0 62.3 1.9 21.0 0.0 0.0 0.8 0.8 Li₂CO₃ Initial stage of the 46SiO₂-rich 43 steel melting process B44 0.55 1.60 0.71 0.003 0.70 — —22.4 39.7 1.7 35.1 0.0 0.0 1.1 1.1 Li₂CO₃ Initial stage of the 36Al₂O₃-rich 30 steel melting process B45 0.63 1.45 0.65 0.003 0.65 — 0.0929.2 30.0 1.5 36.0 0.0 0.0 1.3 1.3 Li₂CO₃ Initial stage of the 35Al₂O₃-rich 34 steel melting process B46 0.65 0.45 0.65 0.004 0.65 0.0916.4 53.1 20.1 9.3 0.0 0.0 1.1 1.1 Li₂CO₃ Initial stage of the 51MgO—SiO₂ 32 steel melting process B47 0.61 2.01 0.90 0.005 0.00 0.2516.0 71.8 2.7 8.6 0.0 0.0 0.9 0.9 Li₂CO₃ Initial stage of the 66SiO₂-rich 45 steel melting process B48 0.60 2.00 0.89 0.005 0.96 0.250.1 53.5 16.9 2.9 23.7 0.0 0.0 1.0 1.0 Li₂CO₃ Initial stage of the 38CaO—SiO₂ 48 steel melting process

As obvious from data on steel wires for forming steel cords shown inTable 3, the number of large inclusion particles of 20 μm or above issmall, the size of the largest inclusion particle is small and thefrequency of breakage of the wires during the wire drawing process issmall when Li is added to the molten steel at a final stage of the ladlerefining process (Samples Nos. A24 to A30). It is known from Table 3that the frequency of breakage of the steel wires made of steelscontaining 0.3 or fewer large hard inclusion particles of 20 μm or aboveper 50 g of steel is obviously low.

The composition of inclusions contained in the steel wires does notconform to a composition specified by the present invention and thefrequency of breakage per 10 t of the steel is greater than twenty timesand the number of large hard inclusion particles of 20 μm or above ishigh when Li is added to the molten steel at an initial stage of theladle refining process (Samples Nos. A31 to A38).

As obvious from data on the steel wires for forming valve springs shownin Table 4, the fracture ratios of the steel wires of sample Nos. B27 toB38 meeting all the requirements of the present invention are relativelylow and the size of the largest inclusion particle contained therein issmall.

The steel wires of sample Nos. B39 to B48 not meeting the requirementsof the present invention, which are comparative examples, haverelatively high fracture ratios and the size of the largest inclusionparticle contained therein is large.

INDUSTRIAL APPLICABILITY

The method of making a high-cleanliness steel according to the presentinvention capable of making a high-cleanliness steel excellent in coldworkability and fatigue characteristics can be advantageously applied tomaking steels for forming high-tension steel wires, very fine steelwires and high-strength springs, such as valve springs.

1. A high-cleanliness steel having a total-Li content between 0.020 and9 ppm by mass and containing 1.0 or below of oxide inclusion particleshaving a major diameter of 20 μm or above in 50 g of the steel.
 2. Thehigh-cleanliness steel according to claim 1, wherein the total-Li/Simass ratio representing the ratio in mass of the total amount of Licontained in the steel to the amount of Si contained in the steel isbetween 1×10⁻⁶ and 1000×10⁻⁶.
 3. The high-cleanliness steel according toclaim 1, wherein each of the oxide inclusion particles has a CaO contentbetween 15 and 55% by mass, a SiO₂ content between 20 and 70% by mass,an Al₂O₃ content of 35% by mass or below, a MgO content of 20% by massor below and a Li₂O content between 0.5 and 20% by mass.
 4. Thehigh-cleanliness steel according to claim 3, wherein each of the oxideinclusion particles has a Li₂O/SiO₂ mass ratio between 0.01 and 0.5. 5.The high-cleanliness steel according to claim 3, wherein each of theoxide inclusion particles has a SiO₂ content of 30% by mass or above andbelow 45% by mass.
 6. The high-cleanliness steel according to claim 3,wherein each of the oxide inclusion particles contains Na₂O and/or K₂Oand the sum of Li₂O content, Na₂O content and K₂O content is between 0.5and 20% by mass.
 7. The high-cleanliness steel according to claim 1,wherein the steel has a C content of 1.2% by mass or below, a Si contentbetween 0.1 and 4% by mass, a Mn content between 0.1 and 2.0% by mass,and an Al content of 0.01% by mass or below.
 8. The high-cleanlinesssteel according to claim 7, wherein the steel has an O content of 0.005%by mass or below, a total-Mg content between 0.1 and 15 ppm by mass anda total-Ca content between 0.1 and 40 ppm by mass.
 9. Thehigh-cleanliness steel according to claim 7, wherein the steel containsat least one of Cr, Ni, V, Nb, W, Cu and Ti.
 10. The high-cleanlinesssteel according to claim 7, wherein the other elements of the steel areFe and unavoidable impurities.
 11. The high-cleanliness steel accordingto claim 8, wherein the steel contains at least one of Cr, Ni, V, Nb, W,Cu and Ti.
 12. The high-cleanliness steel according to claim 8, whereinthe other elements of the steel are Fe and unavoidable impurities. 13.The high-cleanliness steel according to claim 9, wherein the otherelements of the steel are Fe and unavoidable impurities.
 14. Thehigh-cleanliness steel according to claim 1, wherein the steel has atotal-Li content between 0.020 and 6 ppm by mass.
 15. A method of makinga high-cleanliness steel excellent in cold workability and fatiguecharacteristic, said method comprising adding a Li-containing substanceselected from the group consisting of a Li—Si alloy, Li₂CO₃, and acombination thereof, having a Li content between 20 and 40 % by mass, toa molten steel; and producing the steel of claim
 1. 16. The method ofmaking a high-cleanliness steel according to claim 15, characterized byadding a substance containing at least one of Ca, Mg, Na and K to themolten steel in addition to the Li-containing substance.
 17. The methodof making a high-cleanliness steel according to claim 15, characterizedby adding the Li-containing substance to the molten steel after thecompletion of a series of operations of a ladle refining processincluding composition adjustment, temperature adjustment and slagrefining to control the composition of the molten steel such that thesteel has a total-Li content between 0.020 and 9 ppm by mass andcontains 1.0 or below of oxide inclusion particles having a majordiameter of 20 μm or above in 50 g of the steel.
 18. The method ofmaking a high-cleanliness steel according to claim 15, characterized byadding the Li-containing substance at a final stage of a series ofoperations of a ladle refining process including composition adjustment,temperature adjustment and slag refining such that an oxide inclusioncontained in the steel has a CaO content between 15 and 55% by mass, aSiO₂ content between 20 and 70% by mass, an A1 ₂O₃ content of 35% bymass or below, a MgO content of 20% by mass of below and a Li₂O contentbetween 0.5 and 20% by mass.
 19. The method of making a high-cleanlinesssteel according to claim 15, characterized by adding the Li-containingsubstance to the molten steel contained in at least one of a ladle, atundish for continuous casting, and a mold for continuous casting. 20.The method of making a high-cleanliness steel according to claim 15,characterized by adding the Li-containing substance to the molten steelby stirring the molten steel with iron tubular wires containing theLi-containing substance.
 21. The method of making a high-cleanlinesssteel according to claim 15, characterized by adding the Li-containingsubstance to the molten steel by blowing an inert gas carrying theLi-containing substance into the molten steel.